This invention is based upon and claims a priority from Japanese Patent Application No. Hei. 4-273044 filed Oct. 12, 1992 by the present assignee, a disclosure of which is incorporated by reference herein.
This invention relates to a zoom lens system that is suitable for use with compact cameras which have small constraint on back focus. More particularly, the invention relates to a zoom lens system that uses a smaller number of lens elements and which yet features a high zoom ratio of at least 2.5.
Various types of zoom lens systems have heretofore been known for use with compact cameras and they can be categorized in two major classes according to the number of lens elements used; the first class uses at least ten lens elements and features a zoom ratio of about 2.8 (see, for example, Japanese Patent Laid-Open Publication No. Sho 63-43115) and the second class uses a smaller number of elements (6 to 8 lenses) but makes extensive use of aspheric surfaces to feature a zoom ratio of 2.2 to 2.8 (see, for example, Japanese Patent Laid-Open Publication No. Hei 4-78810).
However, these conventional zoom lens systems are still unsatisfactory from the viewpoint of reducing the number of lens elements and, hence, they suffer from the problem of high production cost. Example 5 of the invention described in Japanese Patent Laid-Open Publication No. Hei 3-150518 proposes a zoom lens system that uses only four lens elements and which yet features a zoom ratio of about 2.8. However, this lens system has a very large F number (1:9.6-28.3) and is substantially unsuitable for use with still cameras.